Check the toe-out. Measure from ski to ski 10" in front of the spindle, 10" to the rear of the spindle. I usually use the wearbar as the reference point, as it's consistent and in the middle of the ski. The front measurement should be 1/8" to 1/4" greater than the rear. Any toe in will contribute to darting.
When he replaces the carbides, consider going to dually carbides. They really cut down on darting.